While what you want to do can be done, the main issue you are going to run into is that there are more updates which have to be installed before SP2 for Vista can be installed. Mainly that SP1 must be installed first.
So if you dont have a vista disk that has SP1 already on it you will have to get that installer first, install that, then get SP2, and install that. Which only secures you up to the release of that. It wont include the set of patches for this month for sure which had a reported 31 critical fixes.
So while what you want to do is good. It wont fully secure the system.